1. Field of the Invention
The present invention relates to an optical scanning apparatus for use in image forming apparatuses such as digital copiers and laser printers, and in more particular relates to a multi-beam optical scanning apparatus simultaneously scanning a plurality of scanning lines, and using a semiconductor laser array as a light source in which light emitting sources are arranged in a row, and an image forming apparatus using the multi-beam optical scanning apparatus.
2. Discussion of the Background
Optical scanning apparatuses are used in image forming apparatuses such as digital copiers and laser printers and are widely known. Recently, in these optical scanning apparatuses, optical scanning at a higher writing resolution, e.g., 1200 dpi (dot per inch) or 2400 dpi, is demanded.
Also, as a method of increasing the speed of optical scanning, multi-beam methods where a plurality scanning lines on a scanning surface are simultaneously scanned are gaining attention. For example, a multi-beam optical scanning apparatus using a monolithic laser diode (LD) array or a semiconductor laser array in which light emitting sources are arranged in a row as a light source is now being realized. When such a monolithic LD array is used as a light source, an optical system on a light path from the light source to a scanning surface can be commonly used by a plurality of beams, as in a single-beam optical scanning apparatus using a light source having a single light emitting source. Accordingly, it is possible to realize a multi-beam optical scanning apparatus that is relatively stable against mechanical movements, using such a semiconductor laser array for the light source.
To accomplish optical scanning at a high resolution such as 1200 dpi or 2400 dpi by a multi-beam optical scanning apparatus using a semiconductor laser array, the interval between the light emitting sources of the semiconductor laser array needs to be sufficiently small. For example, when the pitch of a plurality of scanning lines simultaneously scanned by a plurality of beams is a distance corresponding to one scanning line (i.e., as in a so-called adjacent line scanning method), to achieve the writing resolution of 2400 dpi, generally, the interval between light emitting sources of a light source needs to be smaller than 10 μm.
When a monolithic semiconductor laser array is used for a light source, if the interval between the light emitting sources of the semiconductor laser array is smaller than 10 μm, turning on and off of one light emitting source affects turning on and off of an adjacent light emitting source thermally and electrically, so that it is difficult to independently control modulation of each light emitting source. A method for increasing the interval between the light emitting sources of a semiconductor laser array to a certain extent while realizing multi-beam scanning at a relatively high resolution is described for example in Japanese Patent publication No. 2508871.
In this method, the pitch of a plurality of scanning lines simultaneously scanned by a plurality beams is a distance corresponding to more than one scanning lines, i.e., adjacent beams scan a plurality of scanning lines on a scanning surface with a pitch equal to or larger than a distance corresponding to more than one scanning lines. In this case, however, positions where respective beams pass in a scanning optical system are largely separated from each other with respect to the sub-scanning direction. This means that an optical function of the optical scanning system differs for each beam. In particular, a magnification ratio with respect to the sub-scanning direction changes according to an image height of an optical spot, so that a scanning line pitch greatly changes according to the image height. When optical scanning is performed at a relatively high resolution, such a change in a scanning line pitch is relatively large and causes deterioration in the resulting image quality.
As another method of increasing the interval between the light emitting sources of a semiconductor laser array to a certain extent while realizing multi-beam scanning at a relative high resolution, it is proposed to slant the semiconductor laser array to decrease the apparent interval between the light emitting sources of the semiconductor laser array and to simultaneously scan adjacent scanning lines on a scanning surface.
When a semiconductor laser array or an LD array that is slanted as described above is used for a light source of a multi-beam optical scanning apparatus, optical spot diameters are largely influenced by variation in diverging angles of the semiconductor laser array, and the optical spot diameters may be caused to be out of a range of predetermined values for example by environmental changes. When using a semiconductor laser array for a light source in a slanted condition, light quantity on a scanning surface needs to be considered. Further, accuracy in attaching the semiconductor laser array to an optical scanning apparatus greatly influences changes in a scanning line pitch on the scanning surface. Therefore, the accuracy in attaching the semiconductor laser array to the optical scanning apparatus needs to be considered.